The potato is one of the world's most economically important agricultural crop plants. A member of the Solanaceae family, potatoes are conventionally propagated clonally by subdividing tubers, i.e., the underground stems of the plant, into sections which are then planted.
The potato is a premier example of a crop where the control of diseases in the propagation phase is essential for consistent and high yields. The vigor and value of the crop depends in large part on maintaining the source tubers as virus and disease-free as possible. One way of achieving this goal is to produce the certified potato stocks in disease-free areas. However, such environments are not always available.
An alternative to the use of normal tubers for cloning potatoes for production is the micropropagation of microtubers, which are produced in completely disease-free environments. Microtubers are small in vitro produced tubers that are usually about the size of a pea. They are produced in sterile culture under controlled conditions.
Microtubers were first reported in the scientific literature by Barker (1953) Science 118:384-5. Until recently, the expense involved in the production of microtubers prevented the commercial exploitation of microtubers in the potato crop industry. As a result, microtubers were used primarily as a physiological tool to investigate the process of tuberization. Mingo-Castel, et al. (1976, Plant Physiol. 57:480-485) reported on the effects of carbon dioxide promotion and ethylene inhibition on the tuberization of potato explants cultured in vitro. Mauk and Langille (1978, Plant Physiol. 62:438-442) reported on the influence of temperature and photoperiod on the incidence and changes in a cytokinin in a potato plant tissue, and the effect of the cytokinin on in vitro tuberization. Palmer and Smith (1969, Nature 221:279-280) and Stallknecht (1972Plant Physiol. 50:412-413) also reported on the effects of cytokinins and coumarin on in vitro tuberization of potato plants.
Wang and Hu (1982, American Potato Journal 59:33-37) were the first to report on the use of microtubers for the production of potatoes in the field. Subsequently, Wattimena, et al. (1983, American Potato Journal 60:27-33) compared the plant growth, the yield, and the tuber quality of two cultivars of potatoes grown from transplants generated from microcuttings or microtubers with potatoes grown from seed tubers. Both studies used (1) stem cuttings as the explant source for creating microtubers in vitro, (2) Murashige and Skoog (MS) mineral salt medium (Murashige and Skoog, 1962, Physiol. Plant 15:473-497), (3) high sucrose levels (6-8%), (4) low temperatures (15.degree. C.-20.degree. C.), and (5) a synthetic cytokinin to induce tuberization. The procedures differed significantly in the tuberization photoperiod, the method of multiplication, and agitation.
Subsequent publications have been based on both the stationary system (Bourque, et al., 1987, In Vitro Cellular & Developmental Biology, 23/5:381-386; Rosell, et al. 1987, Potato Research 30:111-116; Ortiz-Montiel, et al. 1987, American Potato Journal, 64:535-544; Slimmon and Souza-Machada, 1987, American Potato Journal, 64:458, and Hussey and Stacey, 1984, Annals. of Botany, 53:565-578) varying only in synthetic hormones, the photoperiod and the temperature; and the shaker system (Estrada, et al., 1986, Plant Cell, Tissue and Organ Culture, 7:3-10) where chlorocholine chloride was used in addition to benzyl amino purine (BA) for inducing tuberization.
As mentioned above, the commercial use of microtubers has been limited by the high cost of producing microtubers. This cost is in large part determined by the high input of labor necessary to produce microtubers based on the methods reported above. The labor demands are great because while every node of an in vitro grown shoot has the theoretical potential to form a microtuber, in general, only one microtuber will form on a multi-node shoot axis. Thus, in order to achieve a high rate of tuber formation, the nodes have to be separated from each other by manual manipulation, thus requiring a significant input (i.e., cutting and culturing) for each shoot to culture each microtuber.
An alternative approach to achieving high tuber numbers is the production of a large number of independently-growing shoot axes, each of which will form one or more microtubers. However, this approach has not heretofore been successful on potatoes since the commonly-used hormonal stimulants, such as cytokinins, for shoot multiplication are not particularly effective for potato shoot cultures grown under standard conditions.